Electric heating



Dec. 8, 1959 R. w. SAWYER 2,915,594

ELECTRIC HEATING Filed Aug. 5, 1957 WuHs Densiiy Effecrive Lengflw Inches Fig. 3

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Robert W. Sawyer His AHorney United States Patent O ELECTRIC HEATING Robert Sawyer, Shelbyville, Ind., assignor to General Electric Company, a corporation of New York Application August '5, 1957, Serial No. 676,329

' i i 7 Claims. 01. 219-19 My invention relates to electric heating and more particularly to electric heating elements of the sheathed yp -L In electric heating applications, it is often desired to provide for either a uniform or a predetermined, controlled non-uniform heating. In either case, satisfactory results can be obtained by using a heater device or element designed to have a non-uniform temperature gradient along its heating surface. Such a heater device or element is said to be tapered or, stated otherwise, it is considered to have a graduated thermal characteristic. For example, if it were desired to provide uniform heating, the heater device would be made so as to have a thermal characteristic intended to compensatefor heat losses along its heating surface to surrounding structure and regions not necessarily required 'tofbe heated. In a proper design, the gradient would be selected so that additional heating takes place in those regions where heat losses can be expected to occur. If the gradient is properly selected, the excess heating will substantially equal or compensate for the heat losses, thereby producing a substantially uniform heating as 'desired. Of course, it follows that the gradient can also be selected to provide additional heating in any region to more than equal losses and thereby produce a nonuniform heating as desired.

Heretofore, the so-called tapered heater device or element was obtainedby modifying the heating capacity of distributed parts of the heating element. In one form, the resistance element may have been a continuous wire conductor wound to a uniform outside diameter then subjected to a localized stretching to reduce the number of heat-generating turns in the region selected to have reduced heating. By contrast, other regions were perhaps compressed so as to increase the number of turns therein to thereby increase the heating in those regions.

In another form, the wire heating element was wound so that a region or section thereof had a predetermined reduced diameter in contrast to adjoining sections. Further details of this latter structure may be understood by reference to US. Patent No. 2,499,961, issued to Thomas H. Lennox on March7, 1950.

While the aforementioned practices have produced tapered heating devices for elements having many satisfactory practical applications, it can be appreciated that a certain degree of care would be required in resistance element manufacture and/or electric heater device assembly if an' accurately produced gradient is to be obtained. This is particularly true in the case of a sheathed heater in which a resistance element is positioned within a sheath member and a granular type di-electric, such as magnesium oxide, is compacted therein by tamping and subsequent swaging of the sheath member. The net elfect of these operations may be to cause some localized shifting or possibly elongation of a resistance ele- ,ment within the sheath, thereby modifying the relative of the sheathed heater. Thus, it can be seen that an accurate gradient when desired may not always be easily obtainable.

It is therefore an object of my invention to provide a new and improved heater device having a more easily obtainable and accurately determinable tapered thermal characteristic.

It is also an object to provide an electric heating element in which a tapered thermal characteristic is obtained Without the necessity of prefabricating the resistance element to provide a special configuration.

The objects of my invention are achieved by providing a heating device comprising an electric resistance heating element having a predetermined temperature coelficient of resistance, and heat transmitting means, such as the sheath of a sheath heater, disposed in electrically insulated heat transfer relationship with the heating element and being provided with adjoining heat emitting surface areas of contrasting thermal emissivity characteristics. The heat transmitting means is constructed to have a predetermined thermal conductivity so as to permit the prediction of the character and degree of heat flow between the portions of the heat transmitting means respectively associated with the aforesaid adjacent heat emitting surfaces of contrasting thermal emissivity. The temperature coeflicient of resistance of the heating element and thermal conductivity of the heat transmitting means are selected to be of comparative values which will provide a predetermined variation in heat density between adjacent portions of the heat transmitting means respectively associated with the adjacent heat emitting surfaces of contrasting thermal emissivity.

More specifically, in one aspect of my invention, the heating element is constructed to provide a relatively low temperature coeflicient of resistance, while the heat transmitting means is constructed to provide a relatively high thermal conductivity so as to provide a good heat transfer along the heat transmitting means. In the operation of such a heater, the portion thereof associated with a. heat emitting surface of higher emissivity will tend to be cooler than the portion adjacent thereto and corresponding to an area of lower thermal emissivity, resulting in a heat flow within the heat transmitting means from the hotter portion to the cooler portion, and consequently, there will be a variation in heat density along the heat transmitting means which may be expressed in watts per square inch.

In another aspect of my invention a heating element is selected to provide a relatively high temperature coefiicient of resistance While the heat transmitting means is constructed to provide relatively high thermal conductivity so that there will be relatively poor heat transfer between the adjacent portions of the heat transmitting means respectively associated with adjacent areas of contrasting thermal emissivity. A heater constructed in this latter manner will also provide a tapered heat characteristic desired as will be more fully explained hereinafter.

The novel features which are considered to be characteristic of my invention are set forth with particularity the characteristic of a sheathed heater of the embodiment shown in Figure 2.

Referring to Figure 1 of the drawing, there is shown an electric heater device in the form of a sheathed heater, embodying my invention and comprising a helically wound resistance coil and a tubular metal sheath 11 surrounding the coil. Electric energy for heating the coil is supplied by suitable connections to terminal pins 12 and 13 which are connected in any suitable manner to opposite ends of the coil 10 and are adapted to extend outwardly when in assembled array from the opposite ends of the heating element. In the preferred form, coil 10 is embedded in a mass of compacted di-electric material 14, such as magnesium oxide, thereby electrically insulating coil 10 from sheath 11 while obtaining efficient thermal conductivity between the heat-generating portions of the coil and the sheath. It is, of course, within the purview of my invention that other di-electric materials may be used for embedding the coil and other well-known arrangements for electrically insulating the coil and the sheath may be utilized while maintaining a satisfactory heat-transfer relation, not necessarily conducting.

In the preferred embodiment of the heater of Figure 1, coil 10 is a uniform heat generator throughout its heat-generating extent inasmuch as the coil is uniformly wound, i.e., its turns are evenly spaced and the coil diameter is relatively constant throughout. In addition, and in accordance with my invention the coil is made of a material having a relatively low temperature coefiicient of resistance. In one particular application, satisfactory results were obtained when the coil was wound of an alloy having a temperature coefiicient of approximately 0.00011" C. In the embodiment described, the heat-generating portion of the coil includes all turns not directly connected to the terminal ends although other variations and arrangements may be provided.

The sheath 11 comprising an elongated hollow member in the form of a tubular inner liner or cylinder 15 extending substantially the entire length of and surrounding the coil 10. This inner liner is, in accordance with my invention, fabricated from suitable material having relatively high thermal conductivity, such as copper. Further, in accordance with my invention, the outer surface of the sheath is provided with a plurality of adjoining heat emitting external surfaces having predetermined contrasting thermal emissivity characteristics which in the embodiment of Figure 1, are provided by hollow sleeve members 16, 17, and 18, which are superimposed in any suitable manner in intimate thermal contact with the outer surface of liner 15 and in end-abutting array with each other so as to substantially co-extend over at least the heat-generating portion of coil 10. The outer surfaces of the sleeve members are made to have satisfactory contrasting thermal emissivity characteristics. This may be accomplished in any number of ways, such as polishing or etching, depending on the surface characteristic to be imparted to the sleeve member.

It is of course possible to obtain a satisfactory tapered heat heater in practicing my invention by treating the outer surface of the cylinder 15 directly for purposes of forming the adjoining areas of predetermined thermal emissivity characteristic in which case the provision of the sleeves 16, 17, and 18 is unnecessary. One manner in which this might be done is to etch a portion of the outer surface of the cylinder 15 while other portions are polished, to varying select degrees. Another way in which this may be accomplished is to coat the outer surface of the cylinder 15 with suitable materials in accordance with well-known processes, which coatings may or may not be further etched or polished to obtain a satisfactory surface characteristic suited for the purpose desired. Employing these techniques, it is possible to make a heating element which is designed to have greater heating in the regions proximate its opposite ends as shown in Figure 1. For example, areas corresponding to the sleeves 16 and 18 may be provided with identical coatings of aluminum while the area corresponding to the sleeve 17 may be provided with a satisfactory coating of an iron chrome alloy, such as A1S1 Type 430.

In the embodiment of Figure 2, a sheathed heater having a uniformly wound coil 20 embedded in compacted magnesium oxide 21 is provided with suitable terminals 22 and 23 for connection to an external power supply (not shown). Surrounding coil 20 and the embedding mass 21 is a sheath 24 comprising an elongated hollow member in the form of a cylinder 25. Super imposed on the cylinder 25 in thermal contact therewith proximate the opposite ends thereof and longitudinally spaced apart from each other so as to define an adjoining central longitudinal surface section are sleeve members 26 and 27, respectively.

In the embodiment of Figure 2, the coil 20 is, in accordance with my invention, wound of a wire having a relatively high temperature coefiicient of resistance, and the cylinder 25 is made of material having relatively low thermal conductivity. To obtain the gradient of a heater element designed to produce uniform heating where heat loss at the extremities is expected to occur such as shown in Figure 3, the central longitudinal section is adapted to emit heat at a rate greater than the heat emission of the longitudinal sections at the opposite ends. For this purpose, the outer surface of the cylinder 25 along the central longitudinal section is adapted to have an emissivity characteristic higher than that of the two sleeve members 26 and 27. By way of illustration, satisfactory results to produce the gradient curve such as shown in Figure 3 were obtained where the coil 20 was wound of nickel-iron alloy wire, known in the trade as I-Iytemco, having a temperature coefiicient of resistance approximating 0.0045 C.; the cylinder 25 was made from a nickel-chrome alloy commonly known in the trade as Inconel having a thermal conductivity approximating 104 B.t.u./sq. ft./hr./ F./in. and having its outer surface oxidized so as to have a thermal emissivity characteristic in the range of 0.85-0.95; the sleeve members were made of aluminum, the outer surfaces of which were oxidized to have a thermal emissivity characteristic in the range of 0.11-0.19.

The effect of using the combination of a heat-generating coil 20 of high temperature coefficient of resistance and surrounding it with a sheath having a relatively low thermal conductivity and having its outer surface provided with adjoining areas of contrasting thermal emissivity characteristics is believed to be explainable substantially as follows: Electric current flowing in coil 20 causes the coil to become heated in accordance with well-known principles whereupon heat, according to heat density distribution fixed by the physical distribution of the turns, is transferred to the outer sheath. Because of the relatively high thermal emissivity characteristic of the central section of the sheath, the rate of emission of heat along that section will exceed that of the rate along the surfaces of the adjoining sleeve members. The areas of contrasting thermal emissivity on the sheath may of course be provided by suitably treating the external surface of the cylinder 25 thus eliminating the necessity of providing the sleeves 26 and 27, and if desired, three sleeves may be provided, such as in the construction shown in Figure 1. Because of the relatively poor thermal conductivity of the cylinder 25, a flow of heat from the end sections toward the center section is substantially impeded, thereby preventing a restoration or a tendency to restore uniform heating along the outer surface. Thus, the central section surrounding the central section of coil 20 is subjected to a lower ambient temperature than are the end sections of the coil. Because of this gradient in ambient temperature and thus resistance along the coil 20 the coil is conduced into generating heat in a non-uniform manner along its length.

This can Be'seen 'by refeiencetdFigure 3 where the center sectionofcurve. 30.. defined by. lines 28: and. oug rr r p nds to. the entral P i ation of; the sheathed heater of Figure Zwhereas the gradients along the surface -o'f the opposite end sectionso'f the sheath heater identified-bythe" poi-tions of are curve 3010 the right and .leftoflines, 28, and ,29, respectively, show the end sections operating at a higher watts density than the central section thereby producing the nonuniform thermal gradient desired.

In the manufacture or assembly of a heater device of the type embodying my invention, it is apparent that advantages are obtainable since the assembly of the units making up the heater device can be effectuated prior to providing the outer heating or heat-emitting surfaces with the gradient controlling adjoining areas of contrasting thermal emissivity characteristic. For example, in manufacturing a sheath heater in accordance with my invention, the pre-wound coil may be assembled in the conventional manner within the sheath member and a mass of insulant of granular type may be placed within the space between the sheath and the coil and compacted in accordance with well-known techniques. A second compaction may, if desired, be provided by swaging of the sheath member to the desired reduced diameter. Depending on which technique is to be used in provid ing the sheath member with the plurality of adjoining surfaces, this step may now be taken. For example,

in applying the sleeve members, they may be disposed over the opposite ends and reduced in diameter such as by swaging to effectuate the desired thermal contact.

Therefore, while a particular embodiment of the subject invention has been shown and described herein, it is in the nature of description rather than limitation and it will occur to those skilled in the art that various changes, modifications, and combinations may be made within the province of the appended claims and without departing either in spirit or scope from this invention in its broader aspects.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A tapered heat electric heating device comprising elongated electric heat generating means of the resist ance type having a predetermined temperature coeificient of resistance, heat transmitting means extending along the heat generating means in electrically insulated heat transfer relation therewith and having a predetermined thermal conductivity, the heat transmitting means being provided with a plurality of adjoining heat emitting external surfaces extending along the heat transmitting means in contiguous end-to-end relationship and having contrasting thermal emissivity characteristics, the value of thermal conductivity of the heat transmitting means and the temperature coefiicient of resistance of the heat generating means being selected to be of comparative values to provide a predetermined variation in heat densitybetween adjoining portions of the heating device respectively corresponding to said adjoining heat emitting surfaces of contrasting thermal emissivity.

2. A tapered heat electric heating device comprising an elongated electric resistance heating element having a predetermined temperature coefiicient of resistance, an elongated sheath surrounding the heating element and having a predetermined thermal conductivity, the sheath being in electrically insulated heat transfer relation with the heating element and being provided with a plurality of adjoining heat emitting external surfaces extending along the sheath in end-to-end contiguous relationship and having contrasting thermal emissivity to provide a temperature gradient along the sheath in response to energizing of the heating element, the value of thermal conductivity of the sheath and the temperature coeificient resistance of the heating element being selected to be of comparative values which will provide a predetermined variation in heat density between adjoining portions. of the. sheath. respectively corresponding to said adjoining heat emitting. surfaces of contrast ing thermal emissivity.

3. An electric heating device comprising an elongated electric resistance heating element having a predetermined temperature coeflicient of resistance, an elongated sheath including a hollow metal member surrounding the element in electrically insulated heat transfer relationship therewith and having a predetermined thermal conductivity, and means providing the sheath with adjoining external surfaces of contrasting thermal emissivity spaced longitudinally of the sheath and extending around the heating element including a plurality of additional hollow metal members disposed over the first hollow member and in intimate thermal contact therewith, the temperature coefficient of resistance of the heating element and the value of thermal conductivity of said first hollow member being selected to be of comparative values which will provide a predetermined variation in heat density between adjoining portions of the sheath respectively corresponding to said adjoining surfaces of contrasting thermal emissivity.

4. A tapered heat sheath heater comprising an electric resistance heating element having a relatively low temperature coefficient of resistance, and a sheath surrounding the heating element and being electrically insulated therefrom in heat transfer relationship therewith, the sheath being provided with adjoining heat emitting external surfaces of contrasting thermal emissivity extending along the sheath in end-to-end contiguous relationship and each extending about the heating element, the sheath having a relatively high thermal conductivity to provide good heat transfer between the adjoining portions thereof respectively corresponding to said adjoining surfaces of contrasting thermal emissivity.

5. A tapered heat sheath heater comprising an electric resistance element having a relatively high temperature coefficient of resistance, and a sheath surrounding the heating element and being electrically insulated therefrom and in heat transfer relationship therewith, the sheath being provided with adjoining heat emitting external surfaces of contrasting thermal emissivity extending along the sheath in end-to-end contiguous relationship and each extending about the heating element, the sheath having a relatively low thermal conductivity to provide relatively poor heat transfer between adjoining portions thereof respectively corresponding to said adjoining surfaces of contrasting thermal emissivity.

6. A sheathed heating element comprising an electric resistance heating element having a relatively low temperature coefiicient of resistance, and a sheath for the heating element including a hollow metal member surrounding the heating element in electrically insulated heat transfer relationship therewith and a plurality of sleeve members disposed over said hollow member and cooperating with said hollow member to provide adjoining heat emitting external surfaces on the sheath having contrasting thermal emissivities with each of said surfaces extending around the heating element, said hollow member having a relatively high thermal conductivity to provide good heat transfer between adjoining portions thereof respectively corresponding to said adjoining surfaces of contrasting thermal emissivity.

7. A sheathed electric heater comprising an electric resistance heating element having a relatively high temting external surfaces of contrasting thermal emissivity with each of said surfaces extending around the heating element, said hollow member having a relatively low thermal conductivity to provide relatively poor heat transemissivity.

References Cited in the file of this patent UNITED STATES PATENTS Ruben Mar. 31, 1925 8 Heyroth et a1. Apr. 10, 1934 Trent Apr. 4, 1939 Koci Jan. 18, 1949 Kumpfer Sept. 22, 1953 Rawles Feb. 16, 1954 Fry May 27, 1958 

